The external urethral sphincter (EUS) and external anal sphincter (EAS) are the principal voluntary striated muscles that sustain continence of urine and faeces. In light of their common embryological origin, shared tonic sphincteric action and synchronized electrical activity in vivo, it was expected that they would exhibit similar physiological and structural properties. However, the findings of this study using paired observations of both sphincters isolated from the rat show clearly that this is not the case. The anal sphincter is much more fatigable than the urethral sphincter. On completion of a fatigue protocol, the amplitude of the last twitch of the EAS had declined to 42 +/- 3% of the first twitch, whereas the last twitch of the EUS was almost identical to that of the first (95 +/- 3%). Immunocytochemical detection of myosin heavy-chain isoforms showed that this difference was not due to the presence of more slow-twitch oxidative type 1 fibres in the EUS compared with the EAS (areal densities 4 +/- 1% and 5 +/- 1%, respectively; P = 0.35). In addition, the fatigue difference was not explained by a greater contribution to force production by fast oxidative type 2A fibres in the urethral sphincter. In fact, the anal sphincter contained a higher areal density of type 2A fibres (56 +/- 5% vs. 37 +/- 4% in the EUS, P = 0.017). The higher oxidative capacity of the EUS, measured histochemically, explained its fatigue resistance. These results were surprising because the fatigue-resistant urethral muscle exhibited faster single-twitch contraction times compared with the anal sphincter (56 +/- 0.87 ms vs. 72.5 +/- 1.16 ms, P < 0.001). Neither sphincter expressed the type 2X myosin isoform but the fast-twitch isoform type 2B was found exclusively in the EUS (areal density 16 +/- 2%). The type 2B fibres of the EUS were small (diameter 19.5 +/- 0.4 mum) in comparison to typical type 2B fibres of other muscles. As a whole the EUS is a more oxidative than glycolytic muscle. In conclusion, analysis of the twitch mechanics and fatigue of two sphincters showed that the EUS contained more fatigue-resistant muscle fibres compared with the EAS.